Cats retain their mystery

I just don't understand my cats; what they can do; what they can't (or won't do).
I was curious the other evening because as we were getting ready for bed my wife Tanya and I went through our nightly routine of throwing bck the comforter, blanket and sheet to sweep cat litter from the bed.
BTW: I just learned that Kitty Litter is trademarked, so I can't use it generically.
Anyway, I also noticed they brought food with them to bed this night. That's it! No more. They know beter than that. I got out the tape measure and with the help of my oldest son Brian, came up with what I think are some intersting numbers. From the cat box to the middle of our bed is 38 feet. Nine of those feet are floor tile, which leaves 29 feet of reasonably thick carpet that should capture the litter from their feet.
How is it that they have to travel 29 feet across carpet and still are able to leave litter as an annoying offering?
Now we've got to negotiate with them on brnging food into bed with them. We've got to stand firk on this or we'll be vacuuming the bedsheets every night when we're dog tired. We've got to come to a compromise somehow. I don't hold out great promise.